I have a pack of bolts for the GM300 they are M2 Cap Head with 16mm of thread they are A4 Stainless so not as strong as the original hardened steel ones but they work just fine for me and I have been using the red Modella props I got for John Hook too. You need a small Allen Key to tighten them - 1.5mm measured across the flats
If you still need some let me know and I'll pop a few in the post ( I bought a pack of 50 !)
Chris, if you could spare a couple that'd be great, even if I just keep them as spares. I'm bound to lose the one I've got sooner or later. I'll put the red Modela prop back on for now though as there is no prop jumping issue when I run the motor with that one. It's just the bigger, thicker-hubbed orange Chinese prop (as recommended by Dan) that struggles to stay put with the existing bolt. I haven't tried Jon's Loctite idea yet though, so all may yet be well with that too.